Radio access networks are rapidly becoming increasingly denser and heterogeneous as we move towards 5G. In the future, network architectures will support Heterogeneous Network (HetNet) deployments in which an anchor node provides wide area coverage, signalling and possibly data plane connectivity, whilst subtended small cells offer high bandwidth user plane links to user devices. In this context, a user device can maintain at least two data plane radio links having the possibility to be simultaneously receiving data packets from both nodes.
The anchor node is commonly referred to as the Master eNB (MeNB) since it also provides control plane connectivity for the user device via a Radio Resource Control (RRC) radio link. On the contrary, the small cell nodes are referred to as Secondary eNBs (SeNB) solely providing data plane transmissions. Different realizations of this concept are available in the prior art which are thoroughly described in the following section of this disclosure.
For LTE HetNet deployments, 3GPP LTE Release 12 specifies Dual Connectivity (DC) between macro cell MeNBs and small cell SeNBs operating on different frequency bands and being interconnected via non ideal backhaul (i.e. X2 interface). Specifically, LTE DC is supported by two different network architectures, namely the 3C and 1A architecture.
In the 3C DC architecture a Serving Gateway (S-GW) routes all data packets of a user device Radio Access Bearer (RAB) to the MeNB over the S1 interface. Thereafter, the MeNB is responsible for determining a split of the data packets between the traffic ratio to be sent to the user device via the MeNB and the SeNB, respectively. The packet-level RAB splitting decision takes place at the Packet Data Convergence Protocol (PDCP) layer of the MeNB and PDCP packets are forwarded to the SeNB over the X2 interface. The SeNB queues the received packets and determines when to schedule their transmission to the user device. The weakness of the existing solution is that packet transmissions from the SeNB incur delays of tenths of milliseconds due to the packet forwarding over the X2 interface, a fact that naturally increases the latency of the data plane.
Unlike 3C DC, the 1A DC architecture solely supports RAB splitting at a RAB granularity performed by the S-GW. The S-GW routes each RAB of the user device either to the MeNB or to the SeNB. No packet forwarding from the MeNB towards the SeNB takes place over the X2 interface, and packets belonging to the same RAB are transmitted to the user device only by a single eNB (either the MeNB or SeNB depending on the splitting decision at the S-GW). The 1A architecture is suitable for steering RABs based on their traffic type. For instance, a RAB with strict Quality of Service (QoS) requirements could be routed to the MeNB while best-effort data traffic RAB could be routed to the SeNB. However, the technical problem of this solution is that it limits the aggregated bandwidth allocated to packet transmissions belonging to the same RAB since the user device is only scheduled by a single network node.
In an additional related conventional solution, the LTE SeNB is further co-located with a Wireless Local Area Network (WLAN) access point or Licensed Assisted Access (LAA). The control plane RRC connection is still anchored to the LTE MeNB and the user device can have up to three data plane connections (one with the MeNB and two with the SeNB). Specifically, the architecture reuses the 3C split-bearer concept meaning that the MeNB receives the RAB packet flow from the S-GW; determining a ratio of the packet flow to be forwarded to the SeNB over the X2 interface. Thereby, this conventional solution inherits the drawback of the 3C DC architecture, which is the data plane latency increase owing to the mandatory packet forwarding over the X2 interface.